1. Field of the Invention
The present invention relates to the design of pumping cavities for lasers optimized to reduce thermally induced optical distortions in solid state laser media.
2. Description of Related Art
Lasers using solid state media, such as Nd:YAG, are often implemented with a pumping cavity which encloses both the medium and a lamp providing pumping radiation. Prior art systems of this type are described in U.S. Pat. No. 4,039,970, invented by Shiroki et al., issued Aug. 2, 1977, and in Czarniewski, "Spectral Filter for Nd:YAlG Pumping Cavity", Applied Optics, Vol.10, no.6, June 1971.
Lamps providing pumping radiation, particularly when operated in a pulsed mode, exhibit an output spectrum which typically does not match the fluorescence efficiency spectrum of the laser medium. For example, FIG. 1 shows the spectrum of a xenon flash lamp which exhibits essentially a "black body" radiation profile characterized by high intensity in the ultraviolet range with decreasing intensity toward the infrared end of the spectrum. By contrast, the relative efficiency of fluorescence for Nd:YAG is a function of the excitation wavelength as shown in FIG. 2. It can be seen that the fluorescence efficiency at the UV end of the range is negligible compared with the spectrum above about 700 nanometers. Thus, the high intensity UV radiation supplied by the xenon flash lamp is inefficiently utilized by the laser medium and results in unwanted heat.
As discussed in the Czarniewski and Shiroki et al. references cited above, the UV radiation is known to cause, in addition, solarization in the laser media which results in permanent damage to the crystalline structure, decreasing the output efficiency of the laser. Czarniewski and Shiroki et al. address the solarization problem by placing absorbing filters which remove radiation below about 500 nanometers from the spectrum reaching the laser medium.
The Shiroki et al. reference describes a system in which the laser medium is yttrium-aluminate (YAlO.sub.3). Yttrium-aluminate crystals do not exhibit thermally induced stress birefringence which damages the beam quality.
The filter implemented in the Shiroki et al. reference, therefore, simply addresses the problem of removing UV radiation in order to prevent solarization of the crystal to extend its useful life as a laser medium.
The Czarniewski reference describes a laser using Nd:YAG in the continuous wave mode. Czarniewski inserts an absorbing filter around the laser medium to absorb radiation under about 500 nanometers in order to prevent solarization of the YAG rod. The Czarniewski laser is operated in the continuous wave mode with multiple axial modes, so its output is depolarized and non-Gaussian. Therefore, Czarniewski is not concerned with reducing optical distortions, such as thermally induced stress birefringence in the rod. Furthermore, the pump lamp, when operated in the continuous mode, has an output spectrum which approaches a line source rather than having a "black body" profile as described above with respect to FIG. 1.
CVI Laser Corporation of Albuquerque, NM provides an alternative system for removing UV radiation produced by krypton and xenon flash lamps within laser pump cavities. In the CVI Laser Corporation system, a Pyrex tube with a dielectric coating which reflects UV radiation above 300 nanometers back into the lamp is used. The reflected UV radiation is absorbed by the plasma in the lamp to be re-emitted eventually in the laser pump bands. The Pyrex tube itself absorbs radiation below about 300 nanometers. The CVI Laser Corporation recommends use of the Pyrex tube around the flash lamps in order to reduce solarization of the rod, reduce thermal loading of the rod, and reduce deterioration of the pump cavity due to interaction with contaminants in the cooling system by the UV radiation. However, dielectric coatings are sensitive in their reflectivity to the angle of incidence of the light. So, for light from an extended, uncollimated source such as a flash lamp, they are not very effective. Also, flow tubes of this sort are quite expensive and are not adapted to optimizing control of the thermally induced stress birefringence because of limitations of coating technology.
For pulsed lasers operating with a gaussian or near gaussian transverse mode, stress-induced index of refraction variations and birefringence in the laser medium combine to reduce beam quality. Prior art systems attempt to limit these distortions due to excess heat by flowing an active cooling medium around the rod. However, such systems are ineffective to maintain the high beam quality desired for many pulsed systems.